The present invention concerns piezoelectric resonators and more particularly resonators of small dimensions which are most often used for making frequency generators in particular for portable electronic equipment, in numerous fields such as horology, information technology, telecommunications and the medical field.
Most quartz resonators of small dimensions that are currently used in horology and particularly in electronic or electromechanical watches are quartz tuning fork resonators, which are enclosed in a vacuum or in an inert gas atmosphere in so-called xe2x80x9cmetallicxe2x80x9d cases.
These metallic cases are cases of elongated shape which include a base of essentially cylindrical contour, onto which a metal cap also of cylindrical shape is driven, said case being able to be advantageously manufactured by stamping. In this case, the electric connection between the resonator electrodes and the exterior of the case is assured by two wires or two metal strips which pass through an insulating part of the base and whose inner ends are soldered or glued with the aid of a conductive adhesive onto contact pads of the resonator, so that the wires or strips are also used as a support for the resonator.
Quartz tuning fork resonators have the advantage of being able to be manufactured on a very large scale and at very low cost, by photolithographic techniques, which means that they are also very often used as they are with another type of case, in particular for non-horological applications.
These cases of another type, called xe2x80x9cceramic casesxe2x80x9d, are relatively flat cases which generally include a main part of parallelepiped shape made of ceramic material, inside which the resonator is mounted, a rectangular cover with or without an edge, which can be made of ceramic material, glass or metal and which is soldered via a sealing gasket onto the main part and a connection system which can take different forms for electrically connecting the excitation electrodes of the resonator to the exterior of the case.
Now, the mounting of a tuning fork resonator in this type of case poses problems.
One of these problems is that the dimensional and functional features of the resonators are optimised for mounting them in metallic cases and not in ceramic cases. For example, the ratio between their length and their width is ill suited to the manufacture of such cases, in particular when the cases are of the SMD (Surface Mounting Device) type, i.e. meant to be automatically mounted on hole-free printed circuit boards.
FIGS. 1 and 2 are intended to assist comprehension of other problems posed by mounting a tuning fork in a ceramic case.
These Figures are plan top and bottom views of a conventional tuning fork resonator, like those that are currently used in horology and which are designed to vibrate in flexure mode.
This resonator 2 includes a quartz tuning fork 4 whose base 6 and arms 8 and 10 carry metallisations which form, on the arms, two groups of electrodes 12 and 14 which allow them to be subjected to electric fields to make them vibrate and, on base 6, connection pads 16 and 18 respectively connected to electrode groups 12 and 14.
FIG. 3 is a median and longitudinal cross-section view which shows schematically how resonator 2 of FIGS. 1 and 2 is generally mounted in an SMD type ceramic case 20.
This case 20 of parallelepiped shape includes a main part 22 with a bottom 24 and sides 26 and a cover 28 with an edge 30 which has been vacuum soldered, by heat and pressure, using a solder joint 32 for example made of an eutectic alloy of gold and tin, after resonator 2 has been mounted in main part 22.
This mounting of resonator 2 is achieved by soldering or gluing with a conductive adhesive connection pads 16 and 18 of the back face (see FIG. 2) onto corresponding conductive pads 34 provided on a step 36 of the bottom 24 located on one side of main part 22 of the case, these pads 34 being connected, in a manner not shown in the drawings, to external contact pads 37 and 38 disposed on the back face of bottom 24.
In these conditions, a large part of the resonator overhangs step 36 of the bottom of the case which supports it and this step 36 is relatively far away from the centre of gravity of the resonator.
Because of this, the resonator and its connection to the case do not have very good resistance to shocks.
On the other hand, the resonator is liable to tip towards the bottom of the case when it is fixed onto step 36 of this case.
Further, since the thermal expansion coefficients of ceramic material and quartz are different enough to create mechanical stresses in the resonator which can not only be felt in the arms of the tuning fork and disturb the working of the resonator when the temperature changes but can also break the solder or detach connection pads 16 and 18 of the resonator from those 34 of the case and alter or even cut the electric connection between the resonator electrodes and the external contact pads 37 and 38 of the case.
For the same reason, if there is an incipient fracture on the side of base 6 of the tuning fork where connection pads 16 and 18 are situated, a significant temperature variation can even break the tuning fork.
Finally, when the resonator is vacuum packed, the vacuum is never perfect and the stirring of the atmosphere by the tuning fork arms when the resonator vibrates can modify the operating parameters of the resonator when its arms come close to each other, more so than in the case of a metallic case.
The object of the invention is to provide a piezoelectric resonator and an assembly comprising this resonator enclosed in a case, which enables satisfactory solutions to be brought to the various problems posed by conventional tuning fork resonators.
Thus, the piezoelectric resonator according to the invention which is evidently intended to be mounted in a case and which includes a part in the shape of a tuning fork with two parallel arms connected to each other by a base and carrying electrodes to make them vibrate, these electrodes being connected to connection pads intended to be electrically connected to the exterior of said case, is characterised in that it also includes a central arm attached to said base and located between the arms of the tuning fork shaped part, substantially equidistant from said arms, this central arm having a greater mass than those of the arms of the tuning fork shaped part and said connection pads being carried by this central arm.
Moreover, when the resonator is enclosed in a hermetic parallelepiped shaped case, this case, which includes a main part with a bottom and sides in which said resonator is mounted and a cover fixed to this main part, is characterised in that it includes at least one support attached to its bottom and on which said central resonator arm is fixed.